In any discussion of the ionospheric electron density distribution, it is important to recognize that many experimental methods have been used to arrive at our current understanding. The major ones include: ground-based vertical incidence sounding (VIS), topside sounding using satellite platforms, incoherent backscatter radar, the Faraday rotation and signal delay of satellite signals, as well as in-situ measurements using rocket probes and satellites. The VIS method, which employs the High Frequency (HF) band, was the earliest method and has provided the most comprehensive picture of the lower ionosphere and its worldwide distribution. The ionospheric D layer is an exception, and special methods are needed to determine the electron densities in that region. Much of the current nomenclature involving ionospheric structure and phenomena is a carryover from early VIS investigations. As a consequence we shall briefly examine the VIS method. For a discussion of other methods, the reader is referred to Hunsucker (1991).
The plasma frequency associated with an electron gas, fp (a natural resonant frequency), is proportional to the square root of the electron density of the gas.
where fp is in Hz and Ne is expressed in electrons per cubic meter.
It may be shown that a radiowave, propagating vertically upward into the ionosphere, will penetrate the region until it reaches a point at which the sounding frequency matches the plasma frequency. AH frequencies less than this value will be reflected back to ground. An ionospheric sounder is essentially a radar, which maps out the height-dependent ionospheric electron concentration versus transmission frequency, where the probing frequency is typically a stepwise increasing function of time. A plot of signal echo time delay versus transmission frequency is called an ionogram. A typical ionogram and the corresponding ionospheric profile are given in Figure 3-7.
foF2= 10.95 MHz
foF2= 10.95 MHz
Figure 3-7: A typical vertical-incidence ionosonde recording (i.e., ionogram) and the plasma frequency profile fp(h). The electron density profile is related to the plasma frequency profile by equation 3-9 in the text. This ionogram was obtained from the NOAA-NGDC web site and the sounder instrument was developed by the University of Massachusetts at Lowell for the U.S. Air Force. Information derived from the National Geophysical Data Center, Boulder, CO, NOAA, Department of Commerce.
If Nmax is the maximum electron density of a layer, then we define a so-called critical frequency of reflection, fc, which is the maximum plasma frequency within the layer. If the sounder transmission frequency exceeds fc, then the signal is not reflected and penetrates the layer. There are as many critical frequencies in the ionosphere as there are layers or regions. A more complete treatment of the theory of radio propagation in the ionosphere shows that a magnetoionic medium supports 2 modes of propagation, the so-called ordinary (i.e., O-mode) and the extraordinary (X-mode). These modes encounter slightly different indices of refraction and thus travel with slightly different velocities and directions. As a consequence, each ionogram consists of two traces corresponding to O- and X-mode echoes. These traces may be closely aligned over a large portion of their respective propagation bands but can depart significantly at their respective critical frequencies, with the X-
mode supporting somewhat higher frequency signal reflections. By convention, the O-mode trace is used for conversion of ionogram critical frequencies into maximum electron densities. The following convenient expression is used:
where f„ is the ordinary ray critical frequency (MHz) and Nmax is the maximum electron density of the given layer (electrons/cubic meter). Equation 3.10 is equivalent to Equation 3.9.
From an historical perspective, it is interesting to note that the concept of radar detection of aircraft derived from the early work of ionospheric specialists who were already using ionospheric sounders as a means to "detect" ionospheric layers (see Chapter 1).
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